Apparatus for controlling exposure during photographic printing of variable contrast material



Sept. 29, 1970 BURGER ETAL 3,531,199

APPARATUS .FOR CONTROLLING EXPOSURE DURING PHOTOGRAPHIC PRINTING OFVARIABLE CONTRAST MATERIAL Filed March l9, 1968 5 Sheets-Sheet 1 Fig.7

INVEN TOR THEODOR BURGER ERHARD HELLMIG JACQUES LEON VANHEERENTALS LUCYVES NATENS GERT BREIDENBACH wz dz u' S M Sept. 29, 1970 T. BURGER ETALAPPARATUS FOR CONTROLLING EXPOSURE DURING PHOTOGRAPHIC PRINTING OFVARIABLE CONTRAST MATERIAL 5 Sheets-Sheet 2 Filed March 19, 1968 m a E VD- m a. 91% m? H o o o A/Eb 9 z b @N MR M mm 8 a Q w w @m w fl v I On OnA) (L 0 ownlv E: :::::z ml|ll||qll|lldli+lfi Q 1 N Hi EILMIM! w it? E vNv1 vN New 3 NW N t 2 R R A g Q. 0% W RENTALS Sept. 29, 1970 T. BURGER ETAL APPARATUS FOR CONTROLLING EXPOSURE DURING PHOTOGRAPHIC PRINTING OFVARIABLE CONTRAST MATERIAL Filed March 1.9, 1968 5 Sheets-Sheet 5 Fig.5

IN V EN TOR5 THEODOR BURGER ERHARD HELLMIG JACQUES LEON VANHEERENTALSLUC YVES NATENS GERT BREIDENBACH 5 (AL mat 5 J $116M;

HY? //4 y Sept. 29, 1970 BURGER ETAL 3,531,199

ING PHOTOGRAPHIC APPARATUS FOR CONTROLLING EXPOSURE DUR PRINTING OFVARIABLE CONTRAST MATERIAL I Filed March 19, 1968 5 Sheets-Sheet 4INVENTORFB THEODOR BURGER ERHARD HELLMIG JACQUES LEON VANHEE LUC YVESNATENS.

RENTALS Sept. 29, 1970 BURGER ETAL 3,531,199

APPARATUS FOR CONTROLLING EXPOSURE DURING PHOTOGRAPHIC PRINTING OFVARIABLE CONTRAST MATERIAL Filed March 1-9, 1968 5 Sheets-Sheet 5 Fig.7

IN V EN TOR THEODOR BURGER ERHARD HELLMIG JACQUES LEON VANHEERENTALS LUCYVES' NATENS GERT BREIDENBAC United States Patent 3,531,199 APPARATUSFOR CONTROLLING EXPOSURE DURING PHOTOGRAPHIC PRINTING OF VARIABLECONTRAST MATERIAL Theodor Burger, Munich, and Erhard Hellmig,Leverkusen, Germany, Jacques Leon Vanheerentals, Schoten, and Luc YvesNatens, Berchem, Belgium, and Gert Breidenbach, Munich, Germany,assignors to Agfa- Gevaert Aktiengesellschaft, Leverkusen, Germany FiledMar. 19, 1968, Ser. No. 714,188 Claims priority, application Germany,Apr. 26, 1967, A 55,554 Int. Cl. G03b 27/76 US. Cl. 35571 34 ClaimsABSTRACT OF THE DISCLOSURE Apparatus for controlling exposure duringphotographic printing of variable contrast paper which is exposed tolight passing through a negative in two extreme colors and whosegradation is different for each extreme color comprises an electronictimer wherein a first knob selects the total exposure time as a functionof the density of a negative and by considering the sensitivity ofpaper. A second knob selects the ratio of exposures to light in twoextreme colors within the total exposure time by way of a calibrationcurve which is determined in advance for the particular contrast gradeof paper as a function of the density range of a negative.

BACKGROUND OF THE INVENTION The present invention relates to anapparatus for controlling exposure during the photographic printing ofvariable contrast photofinishing material which is exposed to lightpassing through a negative in two different colors.

In the graphic arts, particularly in the production of intaglio printingforms by photographic means, the contrast of prints must vary within anaccurately determined range. In accordance with presently prevailingpractice, negatives are photographed and the exposure time is selectedas a function of the maximum or minimum density of such negatives. Theresulting prints are developed by regulating the duration of processingand the temperature and/or concentration of developing agent in such away that the developed print exhibits the desired range of densities.Such processes are complicated; also, they consume too much time andmust be carried out by highly skilled technicians. Furthermore, thenumber of exposures which turn out badly is very high. Still further,the just described processes cannot be carried out by resorting tocontinuous developing machines of the type known in X-ray technologywherein the conditions during processing remain unchanged.

It is also known to make prints from amateur photographs by exposingphotographic paper to light in two different color to obtain a desiredgradation. The apparatus which are used for making such prints comprisea fully automatic scanning unit which determines the maximum and minimumopacities of a negative. A computer calculates the exposure time on thebasis of the thus determined density values. The prints are made onvariable contrast (gamma) paper which is exposed to yellow and bluelight. A serious drawback of presently known exposure control apparatusfor variable contrast paper is that it is extremely difficult to changethe ratio of blue light to yellow light for different contrast grades ofprinting material and for different gamma values so as to insuresatisfactory prints for all intermediate values. The relationshipbetween the gamma values for both extreme colors is not linear and nomathematical formula has been 3,531,199 Patented Sept. 29, 1970 devisedto date to reproduce such relationship with a desired degree ofaccuracy. Therefore, the above-outlined conventional exposure controlapparatus failed to gain widespread acceptance in the industry.

It is also known to employ a photographic timing device which can beadjusted to set the exposure time and the desired gradation of printingmaterial in the conventional contrast grades extra hard, hard, etc. Suchtiming device does not establish a fixed relationship between the twoextreme colors and the resulting gamma of printing paper. Accuratedetermination of gamma in accordance with the density range of thenegative is not possible; therefore, such devices cannot be used inreproduction of negatives for graphic arts.

SUMMARY OF THE INVENTION It is an object of the invention to provide arelatively simple but versatile exposure control apparatus which can beused to select the gradation of variable contrast printing material insuch a way that the prints exhibit a standardized range of contrastsirrespective of variations in density of the negatives and by resortingto an unchanged developing procedure.

Another object of the invention is to provide a novel electronic timerdevice which can be utilized in the exposure control apparatus to selectthe ratio of exposures of variable contrast printing paper to light intwo different colors in automatic response to selection of totalexposure time.

A further object of the invention is to provide an exposure controlapparatus which can be used in combination with enlargers, contactprinters, projection printers, and/ or repro cameras.

An additional object of the invention is to provide an apparatus whichcan be used for automatic control of a printer to determine the totalexposure and the ratio of exposures to light in different colors withinthe total exposure time or which can furnish indications to facilitatemanual adjustment of a printer or enlarger to select appropriateexposure times and ratios of exposures to light in different colors.

An additional object of the invention is to provide an apparatus whichcan be used to control exposure of different grades of variable contrastprinting material.

The improved apparatus comprises electronic timer means having firstmovable adjusting means for selecting the total exposure time to lightin both colors (e.g., to blue and yellow light), second movableadjusting means for selecting the ratio of exposures to light in the twocolors within the total exposure time, movable carrier means having acalibration curve which is determined in advance on the basis of thecontrast grade of printing material and as a function of the density ofthe negative, and an opera tive connection between the carrier means andthe second adjusting means to facilitate the selection of the ratio onthe basis of the calibration curve.

The curve enables the person in charge to select the ratio of exposuresto two colors on the basis of a density analysis of the negative in sucha way that the contrast of prints is standardized within a predeterminedrange while the developing treatment to which the exposed variablecontrast is subjected remains unchanged.

It was found that the operation of our timing means is particularlysimple if the setting of the first adjusting means is a function of thepreviously determined maximum density of the negative by resorting to adensity scale which is movable with reference to a stationary index, andif the setting of the second adjusting means is such that thecalibration curve points out that graduation of the scale whichindicates the previously determined minimum density of the negative. Thedensities of the negative can be determined by a suitable densitometer.

The novel feature which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved exposure control apparatus itself, however, both as to itsconstruction and its mode of operation, together with additionalfeatures and advantages thereof, will be best understood upon perusal ofthe following detailed description of certain specific embodiments withreference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of aportion of an exposure control apparatus which may be utilized in thepractice of our method;

FIG. 2 is an enlarged view of certain parts in the apparatus of FIG. 1;

FIG. 3 is a schematic partly sectional view of a lamp housing which canbe controlled by the apparatus of FIG. 1;

FIG. 4 is a diagram of the electric circuit for the apparatus shown inFIG. 1;

FIG. 5 illustrates a portion of a modified circuit;

FIG. 6 is a graph showing certain steps in determination of thecalibration curve;

FIG. 7 illustrates a calibration curve which is determined on the basisof the graph shown in FIG. 6; and

FIG. 8 illustrates a wedge which is utilized in making of test strips.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the housing1 for the electronic timer of an exposure control apparatus whichembodies one form of our invention. This housing does not contain adensitometer which is employed to determine the maximum and minimumdensities of a negative, either by the light reflection or by the lightpenetration method. The densitometer may be of the type which ismanipulated by a technician to measure the density of lightest anddarkest portions of a negative which are spotted by visual observationof the negative, or a fully automatic densitometer which not onlydetects but also records the densities of negatives.

The printing, copying or enlarging machine is also not shown in FIG. 1.Such machine may be a conventional contact-type printer or aconventional projection type photographic printer. The parts in thehousing 1 of FIG. 1 determines the time of exposures in two difierentcolors, namely, the total exposure time and the ratio of exposure timesin two extreme colors. The transition from exposure in one color toexposure in the other color can be controlled directly by the exposurecontrol apparatus. However, and particularly when the exposure times arelong, the apparatus of FIG. 1 may be used to furnish indications of therequired exposure times, and the operator thereupon sets the exposuretimes in the printer or changes the filte'rs in accordance with suchindications. The timer of the exposure control apparatus shown in FIG. 1can be used in connection with conventional densitometers and printers.

The housing 1 resembles a control stand and is provided with a cable 2and plug 13 for connection to an outlet or another suitable source ofelectrical energy. A second cable 3 can be connected to a lamp housingor projector 1a which is shown in FIG. 3. This lamp housing 1aaccommodates electromagnets (not shown) which control a shutter 27 oranalogous light blocking means and the color filter changing mechanism.The lamp housing 1a can be built into a printer or it may constitute anattachment which must be connected to or installed in existing printers.In the latter instance, the light source 25 in the lamp housing 1a ispreferably controlled by the exposure control apparatus.

The front portion of the housing 1 accommodates two adjusting knobs 4, 5one of which can be manipulated by hand to select the positions ofcertain parts in dependency on the minimum density of the negative orpicture carrier and the other of which can be manipulated to select theposition of certain other parts in dependency on the maximum density ofthe negative. A density scale 6 is movable in the housing 1 behind awindow 7. This window 7 further exposes a portion of a calibration curve8. A second window 9 located to the left of the window 7 exposes asecond scale 10. The housing 1 further carries two pilot lamps 11, 12each of which lights up during exposure in one of the two colors, aswitch 14 having a portion movable between the positions I and II, and astarter button 15.

The details of mechanical parts of the timer in the interior of thehousing 1 are shown in FIG. 2. The adjusting knob 5 is fixed to a shaft16 which carries a gear 17 meshing with a toothed rack 18 secured to thedensity scale 6. The rack 18 can move the scale 6 back and forth in thelongitudinal direction of the window 7. The shaft 16 further carries thecontact arms of two potentiometers 19, 69. The adjusting knob 4 issecured to a shaft 20A which carries a bevel gear 21 meshing with abevel gear 22 secured to a rotary carrier drum 23. The gears 21, 22constitute a gear train which establishes an operative connectionbetween the knob 4 and the drum 23. The carrier drum 23. The carrierdrum 23 is rotatable about an axis which is parallel to the window 7 andrack 18 and to the front wall of the housing 1. This drum carries thescale 10* at its left-hand end and it also carries a sheet 8a with thecalibration curve 8. The manner in which the sheet 8a is releasablysecured to the periphery of the drum 23 forms no part of our invention.The sheet 8a may be provided with an imprinted system of coordinates tofacilitate reading of the curve 8. The shaft 20A also carries thecontact arm 20a of a further potentiometer 20. A fixed index or pointer24 is provided on the housing 1 adjacent to the right-hand end of thedrum 23, and this index can register with selected graduations of thedensity scale 6 in response to rotation of the knob 5. A second fixedindex 24a of the housing 1 cooperates with the scale 10 to pinpoint aselected ratio of exposures to yellow and blue light within the totalexposure time.

The lamp housing 1a of FIG. 3 accommodates a printing lamp or ananalogous light source 25 of known construction. It is desirable toselect a lamp whose output is high in the range of the two colors towhich the variable contrast paper is exposed during printing. Lightissuing from the lamp 25 passes through a heat filter 26 and impingesagainst the shutter 27. A filter disk 27a is located between the heatfilter 26 and shutter 27; this filter disk 27a may comprise two layersin order to place the one or the other of two grey filters into the pathof light. The purpose of such grey filters will be described later. Ayellow filter 28 and a blue filter 29 are installed behind the shutter27. These filters can be replaced by filters of other colors as long asthe gamma values of variable contrast paper in the particular colors arewidely different. For example, the yellow filter 28 could be replaced bya red filter. Suitable electromagnets or analogous moving means can beemployed to move the shutter 27, filter disk 27a and filters 28, 29 intoand from the path of light issuing from the source 25. A mirror 30 islocated behind the blue filter 29 to deflect the light by 90 degreesonto the negative or into a copying frame in order to insure homogeneousillumination of the picture carrier.

FIG. 4 illustrates the electric circuit of the exposure controlapparatus. One end of the winding of potentiometer 69 is connected witha source of reference voltage (+15 volts) and the other end of thiswinding is connected to the movable central contact 760 of a two-wayelectric switch 76. The fixed contacts 76a, 76b of the switch 76 areconnected with the ground by way of variable resistors 71, 72. Theseresistors form with the potentiometer 69 a voltage divider for referencepotential. The selected voltage is taken off by the contact arm 69awhich is connected to the input of a buffer amplifier and with themovable central contact 770 of a second two-way electric switch 77. Thefixed contacts 77a, 77b of the switch 77 are connected with the groundby way of variable resistors 73, 74. The parts 69, 71, 72, 73 and 74constitute the elements of a corrective circuit for the Schwarzschildeffects of variable contrast printing paper.

The output potentiometer 19 is mechanically coupled with thepotentiometer 69 as shown in FIG. 2. The contact arm 1% of thepotentiometer 19 is connected with the windings of two additionalpotentiometers 31, 32 which constitute a voltage divider and which areconnected with the ground by way of two resistors 33, 34. The contactarms 31a, 32a of potentiometers 31, 32 are connected with the fixedcontacts 35a, 35b of a third two-way electric switch 35 whose movablecentral contact 350 is connected with the input of an operationalamplifier or signal generator 37 by way of a fixed resistor 36. Afurther resistor 38 is connected to the ground and to a junction betweenthe movable contact 350 and resistor 36.

The operational amplifier or signal generator 37 serves to produce avoltage as a function of time, namely, a negative potential whichincreases from Zero potential at the start of exposure. This amplifier37 comprises an amplifier 39 of the type known from the art of analogcomputers and connected with a feedback element 40. The operationalamplifier 37 produces a voltage which increases above or decreases belowthe starting voltage of the feedback element 40 as a function of time.The plates of the feedback element 40 are connected to theemitter-collector circuit of a switching transistor 41 which dischargesthe feedback element 40 sufliciently in response to a signal from aconventional control unit 42 prior to start of an exposure. Thetransistor 41 is preferably of the silicon junction alloy type. When thetransistor switch is closed, only the saturation voltage across theemitter-collector circuit remains.

The operation of the circuit shown in FIG. 4 is as follows:

When the circuit for control current i is completed, the transistor 41is conductive and the feedback element 40 discharges (current i Thetransistor 41 is assisted by the action of the amplifier 39 because thecontrol current i flows to the input (i When the charge of the feedbackelement 40 is led away, the polarity of voltage is reversed. Thus, aninverse operation of the transistor 41 takes place because i is takenover by i The base current ceases to flow to the input so that thereexists only a saturation voltage of the inversely operating transistorsbetween the inlet and outlet of the amplifier 39. It can be readilyproven that such saturation voltage is in the order of a few millivolts.

The output of the operational amplifier 37 is connected with two currentcomparing or evaluating devices 45, 46 by way of resistors 43, 44. Thesedevices compare the currents flowing from the'operational amplifier 37with reference currents flowing through resistors 47, 48. The referencecurrent for the device 45 is obtained from a source of reference voltage(in the present instance volts) and resistor 47 as a function of theoutput signal of the generator 37. The reference current for the device46 is obtained from the potentiometer (which constitutes a voltagedivider and determines the operating range of the device 46) andresistor 48 which is connected with the contact arm 20a.

The device may be a trigger and can be connected with an electromagnetwhich serves to move the blue filter 29 of FIG. 3 into and from the pathof light issuing from the lamp 25. The device 46 can be connected withan electromagnet for moving the yellow filter 28.

An OR-gate is connected with the current comparing devices 45, 46 tocontrol an electromagnet which moves the shutter 27 from the closed orlight blocking position shown in FIG. 3. The control unit 42 maycomprise an arrangement which moves the shutter 27 to closed posi tionduring exchange of filters 28, 29 in order to prevent unfiltered lightfrom reaching the negative and printing paper if the exchange of filtersrequires a relatively long interval of time.

The exposure control apparatus of FIGS. 1 to 4 operates as follows:

The housing 1 is connected with a source of electrical energy by way ofcable 2 and plug 13 and with the lamp housing 1a of FIG. 3 -by way ofcable 3. The contact arms of potentiometers 31, 32 must be moved topositions in which the voltage at the input of the operational amplifier37 reaches such a value that the rise in voltage at the output of theamplifier 37 as a function of time corresponds to the darkening effectof printing light upon the variable contrast printing paper. It isadvisable to employ two potentiometers (31, 32) because the sensitivityof printing paper (i.e., darkening of paper per unit of time as afunction of intensity of illumination) is normally not the same for bothextreme colors and different contrast grades of printing paper. Forexample, the potentiometer 31 can be used for calibration of exposure inblue light and the potentiometer 32 for calibration of exposure inyellow light. Since the just mentioned calibrations are carried out inthe printer, it is preferred to carry out such calibrations bysimultaneous consideration of the influence of light intensity in therespective color upon the paper. The resistor 38 serves to bring aboutsome linearization of adjustment by potentiometers 31 and 32.

In addition to just mentioned calibration for sensitivity, the operatormust attach to the drum 23 a sheet 8a with a calibration curve 8 whoseconfiguration depends on the gradation of printing paper. The manner ofplotting the curve 8 will be described in connection with FIGS. 6 and 7.

The variable resistors 71, 73 resp, 72, 74 are adjusted to compensatefor Schwarzschild effects of printing paper in the two colors. If thetwo groups of variable resistors 71, 73 and 72, 74 are replaced by fixedresistors, the two-way switches 76, 77 can be dispensed with. Suchmodification of the circuit shown in FIG. 4 can be carried out if theSchwarzschild exponents for the two colors are the same. In suchinstances, the Schwarzschild exponents can be accounted for by alteringthe density scale 6, for example, by pasting a different scale over thescale 6 of FIGS. 1 and 2.

The maximum and minimum opacities of a negative are determined with adensitometer in a manner as described above. The desired maximum andminimum opacities or densities of the print are known in advance. Theoperator then manipulates the adjusting knob 5 to shift the densityscale -6 by way of the gear train 17, 18 and to simultaneously adjustthe potentiometers 19 and 69 so that the index 24 on the housing 1registers with that graduation of scale 6 which corresponds topreviously determined maximum density or opacity of the negative. Thepotentiometer 19 has an antilog characteristic curve and selects for thecalibrating potentiometers 31, 32 a voltage which is indicative of therequired overall exposure time by considering the sensitivity andSchwarzschild exponents of printing paper for the individual colors.Thus, the knob 5 can select the total exposure time. The contact arm 19aof the potentiometer 19 can be said to form part of the adjusting meanswhich includes the knob 5, and movement of the contaetarm 19a by way ofthe knob 5 brings about a change in the position of density scale 6 byway of the gear 17 and rack 18.

In the next step, the operator turns the adjusting knob 4 to select theratio of exposure times in yellow and blue light. The knob 4 is turneduntil the intersection between the calibration curve 8 and density scale6 pinpoints that graduation of scale '6 which indicates the determinedminimum density or opacity of the negative. The index 24a then pinpointson the scale 10 that graduation which indicates the ratio betweenexposures to yellow and blue light within the total exposure time.

It is clear that the mechanism shown in FIG. 2 can be modified so thatthe index 24 pinpoints the graduation corresponding to minimum densityand that the curve 8 pinpoints the graduation corresponding to maximumdensity value of the negative.

The knob 4 also adjusts the potentiometer 2 by moving the contact arm20a to a position corresponding to a value of between ground potentialand volts. The contact arm a can be said to form part of adjusting meansfor selecting the ratio of exposures to yellow and blue light. The gears21, 22 constitute an operative connection between the contact arm 20aand knob 4 on the one hand and the drum 23 on the other hand. When theposition of contact arm 20a corresponds to a potential of. +15 volts,the current comparing device 46 responds almost simultaneously with butalways slightly ahead of the current comparing device 45. The latterterminates the exposure. The exopsure to blue light (filter 29) precedesthe exposure to yellow light (filter 28) and such exposure to blue lightthen takes up almost the entire interval which is allotted for theexposure by appropriate setting of the knob 5. If the contact arm 20a iscloser to a position which is indicative of ground potential (zero pointof current), the exposure to yellow light takes up the major part oftotal exposure time, i.e., the zero point of current at the input of thedevice 46 is reached much sooner. When the device 46- responds, i.e.,when the blue filter 29 is replaced with the yellow filter 28 a suitablerelay (not shown) actuates the switches 35, 7.6 and 77. The resistors71, 73 are disconnected from and the resistors 72, 74 are connected withthe circuit. Also, the voltage divider 31, 33 is replaced by the voltagedivider 32, 34. In this way, the apparatus takes into consideration thesensitivity and the Schwarzschild exponents of printing paper for eachof the extreme colors yellow and blue.

In the next-following step, the operator presses the starter button 15to begin the exposure to blue light (filter 29) so that the pilot lamp11 lights up. When the output current of the operational amplifier 37through the resistor 44 reaches a negative value which equals that ofthe reference current flowing through the resistor 48, the device 46causes the associated electromagnet to remove the blue filter 29 and toclose the shutter 27. At the same time, the aforementioned relayactuates the switches 35, 76 and 77.

Depending on the type of light source, the exposure to the other(yellow) color can begin immediately following withdrawal of the bluefilter 29 or with a short delay which is required to carry out theinterchange of filters. The control unit '42 comprises or controlsconventional means for interrupting or preventing any changes in voltageat the output of the operational amplifier 37 during the exchange offilters. When the exposure is to be resumed (to yellow light), thevoltage at the output of the amplifier 37 changes again and theelectromagnets which move the yellow filter 28 into the path of lightand open the shutter 27 are energized. The exposure to yellow light isterminated when the current flow at the input of the current comparingdevice 45 decreases to zero. The electromagnets are then deenergized sothat the shutter 27 closes and terminates the exposure.

In order to enable the printer to utilize dilferent types of printingpaper (e.g., for print toning or screen technique) without changing theadjustment of calibrating potentiometers 31, 32 and variable resistors71-74, the apparatus preferably comprises two or more sets of suchpotentiometers and variable resistors. The selector switch 14 of FIG. 1can be actuated to connect the appropriate set into the circuit of FIG.4 at the will of the operator,

i.e., depending on the type of paper which is employed in the printer.The position of the filter disk 27a in the lamp housing It: is changedsimultaneously with actuation of the switch14 to place the other greyfilter into the path of light which issues from the lamp 25. This isadvisable in order to select the intensity of printing light independency on the type of printing paper. For example, When operatingwith a color screen, the intensity of light 8 should be much higher thanfor copying of half-tone negatives.

The mechanical construction of the exposure control apparatus can besimplified by fixedly mounting the density scale 6 in the housing 1. Theknob 5 is then used to select the total exposure time in accordance withthe previously determined maximum and minimum opacities of the negativeand the knob 4 is used to determine the duration of exposures to blueand yellow light by simultaneously moving the contact arm 20a of thepotentiometer 20 and by turning the drum 23 with the calibration curve 8to a position in which the curve 8 pinpoints that graduation of thescale 6 which indicates the density range of the negative, namely, thedifference between the maximum and minimum densities. The graduations ofscale 6 then indicate successive values of the density range which mayincrease linearly in a direction from the left to the right, as viewedin FIG. 1 or 2. Such graduations can be applied to astandardized sheetin accordance with the previously described procedure. The manipulationof the just described apparatus is somewhat more complicated because itis necessary to determine not only the maximum and minimum densities ofa negative but also the difference between such extreme densities priorto manipulation of the exposure time selecting knob 5.

FIG. 5 illustrates a modified electric circuit which can replace thecircuit of FIG. 4. The numeral 49 denotes a constant source ofelectrical energy which is employed to charge a capacitor 50 whichreplaces the signal generator 37. The charge of this capacitor 50 ismeasured by two scanning or evaluating circuits 51, 52. The range ofoutput signals produced by circuits 51, 52 can be adjusted by threepotentiometers 53, 54, 55. The scanning circuit 52 is connected with asource 56 of reference potential by way of potentiometer -54 whichlatter is adjustable to account for the sensitivity of printing paper.The scanning circuit 51 receives only a portion of reference potentialby way of the potentiometer 53 which constitutes a voltage divider. Thepotentiometer 55 is in series with potentiometer 53 and is employed totake into account different sensitivities of printing paper to light inthe extreme colors blue and yellow as well as to take into considerationdilferent densities of light which is transmitted to the negative by thelight source. The potentiometer 55 replaces the potentiometers 31, 32 ofFIG. 4. The potentiometers 31, 32 or 55 can be used in addition to or asa substitute for grey filters which may be used in conjunction withcolor filters 28, 29 in order to bring about coarse adjustment of lightdensity at the negative or such adjustment of density that the darkeningof printing paper per unit of time is the same during exposure in eitherone of the two extreme colors. The potentiometer 53 determines the ratioof exposure times in blue and yellow light. A further potentiometer inthe energy source 49 is adjustable to determine the slope of the curvewhich indicates the rise of potential between the plates of capacitor 50as a function of the maximum density of the negative. Otherwise, thecircuit of FIG. 5 operates in the same way as the circuit of FIG. 4.

FIG. 6 illustrates the mode of graphically determining the calibrationcurve 8. The density D of the negative is measured along the abscissa,such density corresponding to the logarithm of [.t wherein I is theintensity of illumination and t is the exposure time. The density D ofthe print is measured along the ordinate. The exposure time is selectedin such a way that the density of the print is in a medium densityrange. The numerals 57 to 62 indicate a bundle of darkening curvesmeasured on prints taken with grey filters with difierent ratios of blueand yellow light. The two outer curves 57, 62 respectively indicateillumination exclusively with yellow and blue light. During calibrationwith potentiometers 31, 32 on the basis of shots taken with grey wedgesin pure yellow and blue light, the curves 57 and 62 are displaced insuch a way that they intersect each other at or close to the point 63.This point corresponds to a print density of 0.3 and is determined bythe original illumination and by calibration for a number 2 density ofnegative. It can be readily proven by calculation that the calibrationcan bring about parallel displacement of curves 57-62 in twice thelogarithmic ratio and that it also applies for other I.t values whichdeviate from the negative density value of 2.0. The curves 5861 indicatea series of changes in the ratio of blue and yellow light, always by 20percent. Each of these curves intersects a diiferent portion of ahorizontal line which intersects the paper density value 1.7 on theordinate.

The curve 8 of FIG. 2 can be obtained in accordance with FIG. 7 byplotting the values indicating the ratio of individual exposures (curves57-62) on a chart indicating the density range of the negative, forexample in the density range of 0.3 to L7. FIG. 7 shows a curve 8 whichindicates that, at a ratio of 40 percent blue light to 60 percent yellowlight, the density range of the negative must be 1.13 in order to obtaina desired density range of 1.4 in the print. If the ratio of blue lightto yellow light is 80 to 20 percent, the density range of the negativeshould be 1.5. On the basis of the curve 8, appropriate adjustment ofthe knob 4 can select any one of a number of intermediate values withoutfurther calculations.

In actual practice, the apparatus is preferably calibrated by resortingto a calibrating wedge. The constant of the wedge equals the constant ofthe scale 6, i.e., D=0.2 cm. In order to simplify the calibratingoperation, the wedge is preferably provided with a small auxiliary scalewhich is printed onto the material which is exposed to light duringcalibration. FIG. 8 shows a calibrating wedge 80 which is provided withan auxiliary density scale 81. The range of graduations on the scale 81is from +0.3 to +0.3 and the zero value coincides with the density value2 of the wedge. The knob 5 is manipulated to place the graduation 2 onthe density scale 6 into registry with the index 24. In order to exposea first test strip, the knob 4 is adjusted first for exposure to yellowlight along and thereupon for exposure of a second test strip to bluelight. A densitometer is then employed to determine the position of thepoint corresponding to desired minimum density (for example, 0.3) oneach of the thus exposed test strips. If the position of such pointcoincides with zero graduation of the auxiliary scale 81, thecalibration of the apparatus for sensitivity is accurate. However, ifthe thus determined points coincide with a positive or negativegraduation of the auxiliary scale, the corresponding calibratingpotentiometer 31 or 32 must be adjusted in accordance with the thusdetermined value. To this end, the potentiometers 31, 32 are providedwith scales having graduations from +0.3 to -0.3 whereby the zerograduation corresponds to the central position of the respective contactarm.

The calibration can also be carried out in such a way that the curves57-62 of both grey filters intersect each other on the line passingthrough the graduation 1.7 on the ordinate. It is further clear that therange +0.3 to -0.3 of graduations on the auxiliary scale 81 and thevalue 2 for the maximum density are given solely by way of example.

Our exposure control apparatus can also be used in combination with anenlarger. The lamp housing 1a of FIG. 3 is then replaced by a box whichis placed in the path of light in an enlarger and contains only thecolor filters 28, 29 and shutter 27. The mirror 30 is omitted and thelamp 25 is replaced by the light source of the enlarger. The function ofthe grey filter 27a is performed by the diaphragm in the enlarger. Whenthe rate of magnification is changed, the diaphragm is adjusted tomaintain the intensity of light at a substantially unchanged value.

If the apparatus is used in combination with a repro camera having amotor-driven rotary filter disk and which can contain a number offilters, an adapter must be connected between the camera and theexposure control apparatus in order to convert the signals produced byapparatus for direct control of electromagnets which move the shutter 27and filters 28, 29 into appropriate electrical impulses serving toregulate rotation of the filter disk and to insert selected filters intothe path of light in the camera.

It is further clear that the apparatus is not limited to a control ofthe exposure of printing material whose graduation is different for blueand yellow light. For example, the apparatus can control exposure ofprinting material in blue and red light. The thus exposed materials canbe processed under identical circumstances regarding the duration ofdeveloping step and the temperature, type and concentration ofdeveloping solution. Very satisfactory results are achieved if the teststrips and the exposed printing material are developed in accordancewith gamma-infinity, namely, if they are developed for such periods oftime that the developing solution ceases to cause further darkening.Such method of developing can be carried out by semiskilled personswithout resorting to expensive automatic processing machines.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of our contribution to the art, and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. In an apparatus for controlling exposure during the photographicprinting of variable contrast printing material which is exposed tolight passing through a negative in two colors and whose gradation isdifferent for each of said colors, a combination comprising electronictimer means having first movable adjusting means for selecting the totalexposure time to light in said colors; second movable adjusting meansfor selecting the ratio of exposures to light in said colors Within saidtotal exposure time; movable carrier means having a calibration curvedetermined on the basis of the contract grade of said printing materialand as a function of the density of the negative; and an operativeconnection between said carrier means and said second adjusting means tofacilitate the selection of said ratio on the basis of said curve.

2. A combination as defined in claim 1, wherein said timer means furthercomprises movable graduated density scale means coupled with said firstadjusting means for movement relative to fixed index means, said totalexposure time being selected when said index means pinpoints on saidscale means a graduation which is indicative of one extreme densityvalue of the negative, said second adjusting means being arranged tomove said carrier means with reference to said scale means and saidratio being selected when said calibration curve pinpoints on said scalemeans that graduation which is indicative of the other extreme densityvalue of the negative.

3. A combination as defined in claim 2, further comprising a grey filterwhich is utilized in determination of said calibration curve, saidfilter having second density scale means whose constant is the same asthat of said first mentioned scale means.

4. A combination as defined in claim 3, wherein the constant of saidfirst mentioned scale means is alterable as a function of theSchwarzschild exponents of variable contrast printing material.

5. A combination as defined in claim 2, wherein said first adjustingmeans comprises a potentiometer having an antilog characteristic curvesaid potentiometer including a movable portion and further comprising agear train for changing the position of said scale means in response tomovement of said movable portion.

6. A combination as defined in claim 2, wherein said second adjustingmeans comprises a potentiometer having a movable contact arm and whereinsaid carrier means comprises a drum rotatable about a predeterminedaxis, said operative connection being provided between said drum andsaid contact arm.

7. A combination as defined in claim 6, wherein said operativeconnection comprises a gear train.

8. A combination as defined in claim 6, further comprising a housing forsaid timer means, said housing having a front wall and said axis beingparallel to said wall.

9. A combination as defined in claim 6, wherein said drum is providedwith a scale which indicates the ratio of exposures to light in saidcolors in each angular position of the drum.

10. A combination as defined in claim 2, wherein said first adjustingmeans comprises a potentiometer including a movable portion and saidsecond adjusting means comprises a voltage divider having a movableportion, said operative connection being provided between the movableportion of said voltage divider and said carrier means, said timer meansfurther comprising means for changing the position of said scale meansin response to movement of the movable portion of said potentiometer,generator means for producing a signal as a function of time which isvariable by said potentiometer, first evaluating means for said signal,said first evaluating means having an operating range which is variableby said voltage divider and being arranged to effect a change fromexposure of printing material to light in one of said colors to exposureto light in the other color, and second evaluating means arranged toterminate the exposure of printing material to light in the other colorin dependency on the signal produced by said generator means.

11. A combination as defined in claim 10, wherein said generator meanscomprises a capacitor and a source of electrical energy connected incircuit with said capacitor.

12. A combination as defined in claim 11, further comprising a pluralityof variable resistors connected in series with said voltage divider toadjust said evaluating means as a function of sensitivity of printingmaterial to light in said colors.

13. A combination as defined in claim 10, wherein said generator meanscomprises an analog computer amplifier circuit, a feedback element forsaid amplifier circuit, and short-circuiting means in parallel with saidfeedback element and arranged to open at the start of an exposure.

14. A combination as defined in claim 13, wherein said feedback elementis a capacitor.

15. A combination as defined in claim 13, wherein said short-circuitingmeans comprises a transistor having an emitter-collector circuit inparallel with said feedback element and a base, said timer means furthercomprising a control circuit connected with said base and arranged toproduce a signal to change the condition of said transistor on thecompletion of an exposure.

16. A combination as defined in claim 13, wherein said potentiometer hasan antilog characteristic curve and is connected in circuit with saidamplifier.

17. A combination as defined in claim 10, wherein said timer meansfurther comprises a corrective circuit for the Schwarzschild eifects ofprinting material, said corrective circuit including a secondpotentiometer having a linear characteristic and including a movableportion mechanically coupled with the movable portion of said firstmentioned potentiometer.

18. A combination as defined in claim 17, wherein said correctivecircuit further comprises at least one variable resistor in series withsaid second potentiometer.

19. A combination as defined in claim 18, wherein the movable portion ofsaid second potentiometer supplies voltage to said first mentionedpotentiometer.

20. A combination as defined in claim 19', wherein said timer meansfurther'comprises a buffer amplifier connected between the movableportion of said second potentiometer and one terminal of the winding ofsaid first mentioned potentiometer.

21. A combination as defined in claim 20, wherein said butter amplifierhas a high input impedance and a low output impedance.

22. A combination as defined in claim 17, wherein said correctivecircuit further comprises at least one resistor connected between themovable portion of said second potentiometer and the ground to enhancethe corrective eflect of said second potentiometer.

23. A combination as defined in claim 10, wherein said timer meansfurther comprises a source of D-C potential and wherein saidpotentiometer constitutes a voltage divider and is connected betweensaid source and the ground. 24. A combination as defined in claim 23,wherein said timer means further comprises a pair of additionaladjustable voltage dividers receiving current from the movable portionof said potentiometer, said additional voltage dividers being connectedin parallel with the ground, said additional voltage dividers havingmovable portions connectable with the input of said signal generatormeans.

25. A combination as defined in claim 24, wherein said timer meansfurther comprises two-way switch means for connecting said input withthe movable portion of one of said additional voltage dividers at atime.

26. A combination as defined in claim 25, wherein said switch meanscomprises a pair of fixed contacts each connected with the movableportion of one of said additional voltage dividers and a third contactmovable into engagement with one of said fixed contacts at a time andconnected to said input, said timer means further comprising a resistorconnected between said movable contact and the ground to linearize theadjustment by said additional voltage dividers.

27. A combination as defined in claim 10, wherein said signal generatingmeans comprises an amplifier having an output connected with saidevaluating means and wherein said second evaluating means is arranged toreceive a reference signal and to terminate the exposure in response toa predetermined relationship between said first mentioned signal andsaid reference signal.

28. A combination as defined in claim 10', wherein said generator meanshas an output connected with said first evaluating means and whereinsaid voltage divider is arranged to supply to said first evaluatingmeans a portion of a reference current producing a second signal, saidfirst evaluating means being arranged to effect said change when themagnitude of said first mentioned signal is in a predelterminedrelationship to the magnitude of said second signa 29. A combination asdefined in claim 10', further comprising a pair of resistor meansconnected between the output of said generator means and said evaluatingmeans.

30. A combination as defined in claim 29, wherein the resistance of saidresistor means is identical.

31. A combination as defined in claim 29, wherein the resistance of oneof said resistor means is a predetermined fraction of the resistance ofthe other resistor means.

32. A combination as defined in claim 10, further comprising resistormeans connected between each of said evaluating means and a source ofreference current.

33. A combination as defined in claim 1, wherein said timer meanscomprises a plurality of adjustable electrical components and whereinsuch components are connected in several groups each of which isutilized for a given type of printing material, and further comprisingchangeover switch means for connecting a selected group into the circuitof said timer means.

34. A combination as defined in claim 33, further comprising a lightsource and a filter element having a plurality of grey filters movableinto the path of light which issues from said source, and furthercomprising means 13 14 for moving a selected grey filter into said pathin response NORTON ANSHER, Primary Examiner to actuation of said switchmeans. L MOSES Assistant Examiner References Cited UNITED STATES PATENTS5 2,462,340 2/1949 Simmon et a1. SSS-37 U.S. Cl. X.R.

